This
Exercise is used to strengthen the rectus abdominus. The rectus
abdominus is responsible for the striated look of the abdominals
known as the 'six pack'. Proper execution of this exercise
requires a slow controlled movement with full extension and
a tight contraction. The obliques and transverse abdominus
play a big supporting role in this movement as well. Make
sure to use a ball that allows your knees to be at a 90 angle
or less. Using a ball on the smaller side is safer than using
one that is to big.

Starting
position: Lie on the ball with your feet pressed into
the ground and knees bent at about 90°.